1. Field of the Invention
The present invention relates to heat transfer printers, and more particularly to heat transfer printers adapted to cover images with a transparent overcoat material.
2. Description of the Related Art
Heat transfer printers for forming color images are conventionally provided. It is practice with such heat transfer printers to cover color images with a transparent overcoat material to prevent fingerprints from adhering to the color image, protect the color image from exposure to chemicals or the like, or preclude the discoloration of the color image due to light.
A description will be given of the operation of the heat transfer printer from the formation of a color image to the transfer (adhesion) of an overcoat material onto the color image formed.
With reference to FIG. 1, an ink film 5 for use in the heat transfer printer has yellow (Y), magenta (M) and cyan (C) color ink faces and a face of overcoat material (O). These color ink faces and overcoat material face are arranged in this order repeatedly to constitute a roll of ink film 5. The ink film 5 and recording paper are nipped between a thermal head and a platen roller, and the ink or the overcoat material is transferred from the ink film 5 onto the recording paper by applying a voltage to the thermal head and thereby heating a plurality of heaters provided inside the head.
In FIG. 1, the region A indicates the width of the recording paper. The region B, which is smaller than the region A, stands for the transfer region of the Y, M or C color ink face. The region C, which is intermediate between the regions A and B, indicates the transfer region of the overcoat material.
When a color image is to be formed by the heat transfer printer, electric power to be supplied to the heaters inside the thermal head is controlled first based on image data sent to the printer as to yellow for every line (pulse data indicating which dots are to be colored and what tone of color is to be given). The power to be supplied to the heaters is controlled, for example, by pulse width control. More specifically, a voltage is applied to the heaters corresponding to particular dots to be colored yellow, with the color tone adjusted by varying the duration of voltage application.
The pulse voltage controlled based on the image data for every line is applied to the specified heaters inside the thermal head, whereby the ink at the portions of the yellow (Y) ink face which are in contact with the energized heaters is adhered to the recording paper. Transport of the paper line by line produces a yellow print. The operation described is similarly performed also for the ink faces of magenta (M) and cyan (C) to form a color image. The overcoat material, which is transparent, is subsequently transferred onto the color image on the recording paper by the thermal head.
FIG. 2 shows the overcoat material in contact with the surface of the color image 14. The overcoat material comprises a thermoplastic overcoat layer 12 formed beneath substrate 10. The overcoat layer 12 is peeled off the substrate 10 by the heat applied to the thermal head and transferred to the surface of the color image 14 on the recording paper.
The state in which the overcoat layer 12 is peeled from the substrate 10 is influenced by the quantity of heat produced by the thermal head, in other words, by the quantity of heat given to the overcoat material. When the quantity of heat produced by the thermal head 20 is proper, the overcoat layer 12 is neatly peeled off the substrate 10, forming a smooth and very glossy surface over the color image 14 upon transfer, as shown in FIG. 3 (a). On the other hand, if the thermal head 20 produces an excessively large quantity of heat, the overcoat layer 12, although transferred onto the color image 14, becomes rough-surfaced owing to thermal deterioration, as seen in FIG. 3 (b), loses gloss from its surface. If producing too small a quantity of heat, the thermal head 20 encounters difficulty in completely transferring the overcoat layer 12 onto the color image 14. Failing to neatly peel the layer 12 from the substrate 10, the overcoat layer 12 transferred to the surface of the color image 14 is locally torn off, as shown in FIG. 3 (c).
In order to neatly transfer the overcoat layer 12 onto the color image 14, it is accordingly necessary for the thermal head 20 to produce a quantity of heat larger than is needed for completely transferring the overcoat layer 12 onto the image 14 but less than a quantity that will thermally deteriorate the surface of the overcoat layer 12 as transferred. When the overcoat material is to be heat-transferred by the conventional heat transfer printer, a pulse voltage is determined and then applied to the thermal head 20 which voltage is optimum to give a quantity of heat fulfilling the above requirement.
However, in the case where the overcoat material is transferred by applying the optimum pulse voltage determined, it is likely that the overcoat material, failing to peel off the substrate 10 neatly, will not be completely transferred to the recording paper at the end portions of the overcoat material transfer region C (see FIG. 1), i.e., at the boundary portions between the overcoat material transfer portion and the nontransfer portion. This is attributable to the quantity of heat of the thermal head 20 at the portions thereof corresponding to the transfer boundary portions of the overcoat material.
Referring to FIG. 4 (a), the conventional thermal head 20, comprises a plurality of heaters 21 arranged in corresponding relation with the overcoat material transfer region C shown in FIG. 1, and a head base plate 22 provided outside the transfer region C to hold the heaters 21. For the heat-transfer of the overcoat material to the surface of the color image 14, all the heaters 21 usually produce heat, so that the distribution of heat from the heaters 21 corresponding to the end portions E of the overcoat material transfer region C to the head base plate 22 exhibit a gentle curve, as shown in FIG. 4 (b).
Stated more specifically with reference to FIG. 4 (b), the end portions E of the overcoat material transfer region C are heated to a temperature range higher than a temperature T1 at which the overcoat layer 12 can be adhered to the surface of the color image 14 and lower than a temperature T2 at which the layer 12 can be reliably adhered to the color image surface, i.e., a temperature range Y in which the adhesion of the overcoat layer 12 to the image surface is unstable. Consequently, the overcoat layer 12 locally fails to neatly peel off the substrate 10 for complete transfer at the end portions E of the overcoat material.
In the case where the overcoat material fails to neatly peel off the substrate 10 at the boundary portions, the transfer boundary portions 8a are torn off raggedly and transferred in a faulty state instead of becoming straight, as shown in FIG. 5. As a result, the color image at the boundary portions is liable to become impaired by fingerprints, chemicals or light because of incomplete transfer of the overcoat material at these protions. If the overcoat layer 12 is torn off from the substrate 10 instead of being neatly peeled off, another problem is encountered in that minute fragments of the overcoat material are created to adhere to another sheet of recording paper subsequently transported, with the result that the image to be printed will be partly left unprinted on the subsequent sheet. Furthermore, fragments of the overcoat material will adhere to the surface of the paper discharge roller, giving rise to the problem of faulty transport of paper. Another problem also arises in that such fragments impair the aesthetic appearance of prints.